Fuel synthesis device

ABSTRACT

A fuel synthesis device includes: a supplier to supply CO2 and H2 gasses; a fuel synthesis catalyst to chemically react the CO2 and H2 gasses to synthesize fuel; a gas-liquid separator to liquefy the fuel into liquid and separate the liquid from a gas containing unreacted CO2 and H2 gasses, and CH4 gas as a side product; a return path to return the separated gas to a point between the supplier and the fuel synthesis catalyst; a bypass path to bypass, and merge downstream of, the return path, and to include a CH4 separator to separate the CH4 and a CH4 oxidation catalyst to oxidize the CH4; and a switching valve to selectively switch between communication with the return path and communication with the bypass path, wherein whether the switching valve communicates with the return path or bypass path is controlled based on the density of CH4.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to Japanese Patent Application No. 2022-003387 filed on Jan. 12, 2022, the disclosures of all of which are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

The present invention relates to a fuel synthesis device to synthesize fuel from hydrogen and carbon dioxide.

BACKGROUND OF THE INVENTION

In order to reduce a negative impact on the global environment, regulation has been tightening on exhaust gas from vehicles. The FT (Fischer-Tropsch) method is widely known as a technique to synthesize gasoline as a fuel from hydrogen (H₂) and carbon dioxide (CO₂) which is contained in exhaust gas and the atmosphere. The FT method is used to synthesize hydrocarbon from carbon monoxide (CO) and H₂, with catalytic reaction. The synthesis of hydrocarbon with the FT method is a kind of polymerization reaction in which carbon chain grows through the reaction. In the FT method, the following reaction occurs (Equation (1) below).

nCO+(2n+1) H₂→C_(n)H_(2n)+nH₂O   (1)

Conventionally, when gasoline (hydrocarbon with 5 carbons or more (C₅₊)) is synthesized with the FT method, CO₂ contained in exhaust gas or in the atmosphere has been converted to CO by a reverse aqueous gas shift reaction (Equation (2) below) and then used in Equation (1) above.

CO₂+H₂→CO+H₂O   (2)

Much research has been done recently to improve efficiency of the FT method, to propose direct synthesis (Direct-FT) as an innovative technique (see Hiroaki Ishizuka, “Commencing research and development of integrated process technology for producing liquid synthetic fuel from CO2,” [online], Feb. 22, 2021, New Energy and Industrial Technology Development Organization, [retrieved on Dec. 20, 2021], Internet <URL: https://www.nedo.go.jp/news/press/AA5_101410.html>, for example). Direct-FT implements the reverse shift reaction and FT synthesis reaction simultaneously in the presence of a catalyst, to produce hydrocarbon directly from CO₂ and H₂ (Equation (3) below, FIG. 3 ). Note that FIG. 3 shows fuel (gasoline) synthesis mechanism in the Direct-FT.

nCO₂+mH₂→C_(n)H_(2(m−2n))+2nH₂O   (3)

As shown in FIG. 3 , CO₂ is transformed into intermediates of CO and CH₂ by H₂ on the surface of a catalyst (e.g., Na—Fe₃O₄/HZSM-5 catalyst), and is then formed into hydrocarbon of C₅₊ (gasoline). In the process, H₂O is produced as a side product. In addition, hydrocarbon of 4 or less carbons (C₁₋₄), including methane (CH₄), is produced as a side product at low temperatures or under lean H₂ conditions, due to variation in temperature regulation associated with instability of feed rates.

The FT method or Direct-FT causes a carbon chain to grow by circulating hydrocarbon of C₁₋₄ and reacting them again with a catalyst to form hydrocarbon of C₅₊. However, hydrocarbon of C₁₋₄, especially CH₄, has low reactivity to cause a carbon chain less likely to grow, so that circulation just keeps density of said hydrocarbon increasing in a reaction system (within a device).

CH₄ gas has a high Global Warming Potential (GWP) and should not easily be released into the atmosphere. CH₄ can be used for city gas, but this requires further refining at refineries or the like and cannot be easily done. In order to reduce the negative impact on the global environment, it is preferable to effectively utilize CH₄ in addition to CO₂.

Under these circumstances, a technique is disclosed in which the CH₄ produced in an FT synthesis reactor by the FT method is sent to a reforming reactor to have a reforming reaction between CH₄ and water vapor (H₂O) through the action of a reforming catalyst, so as to be converted into H₂ and CO (see Japanese Patent No. 6097828, referred to as Patent Document 1 hereinbelow, for example). Note that Patent Document 1 describes that the H2 obtained by the conversion is fed to an FT synthesis reactor so as to be used as a feedstock for hydrocarbon of C₅₊. Patent Document 1 meanwhile describes that CO obtained by the conversion is fed to the reforming reactor so as to be used as an auxiliary fuel for the reforming reactor.

SUMMARY Problems to be Solved

The technique described in Patent Document 1 supplies CO to the reforming reactor, so that the amount of C in the reforming reactor continues to increase. Meanwhile, C in the feed gas supplied to the FT synthesis reactor must be newly supplied separately. In other words, the technique described in Patent Document 1 has a room for improvement in terms of effective utilization of hydrocarbon of C₁₋₄ generated as a side product, particularly CH₄.

The present invention has been made in view of above-described situation and is intended to provide a fuel synthesis device to synthesize fuel by effectively utilizing CH₄ produced as a side product.

Solution to Problem

To solve the above-identified problem, a first aspect of the present invention provides a fuel synthesis device including: a supplier arranged upstream of a main path and configured to supply CO₂ gas and H₂ gas; a fuel synthesis catalyst located downstream of the supplier and configured to chemically react the CO₂ gas and the H₂ gas to synthesize fuel; a gas-liquid separator arranged downstream of the fuel synthesis catalyst and configured to liquefy the fuel into liquid and separate the liquid from a gas containing the CO₂ gas and the H₂ gas, which have not yet reacted with the fuel synthesis catalyst, and gas of CH₄ as a side product; a return path configured to return the gas separated by the gas-liquid separator to a point between the supplier and the fuel synthesis catalyst; a bypass path configured to bypass, and merge downstream of, the return path, and including a CH₄ separator to separate the CH₄ and a CH₄ oxidation catalyst to oxidize the CH₄ separated by the CH₄ separator; a switching valve provided at a junction of the return path and the bypass path and configured to selectively switch between communication with the return path and communication with the bypass path; and a CH₄ density detector arranged in the return path and configured to detect density of CH₄ contained in the gas separated by the gas-liquid separator, wherein whether the switching valve communicates with the return path or the bypass path is controlled based on the density of CH₄ detected by the CH₄ density detector.

Thus, the first aspect of the present invention controls the switching valve based on the density of CH₄, staying in the fuel synthesis device, to cause CH₄ to be partially oxidized and return gas (CO) produced by oxidation to the point upstream of the fuel synthesis catalyst (between the supplier and the fuel synthesis catalyst). This allows the first aspect of the present invention to use CO as a feedstock in the fuel synthesis catalyst to synthesize fuel (hydrocarbon of C₅₊) for sufficiently growing carbon chain.

A second aspect of the present invention provides the fuel synthesis device according to the first aspect, wherein the switching valve is controlled to communicate with the bypass path, on the condition that the density of CH₄ detected by the CH₄ density detector becomes equal to or greater than a predetermined value at which synthesis of the fuel is blocked.

The second aspect of the present invention thus causes the CH₄ remaining in the fuel synthesis device to be partially oxidized and returns the gas (CO) produced by oxidation to the point upstream of the fuel synthesis catalyst, on the condition that density of the CH₄ increases to become equal to or greater than the predetermined value at which synthesis of the fuel is blocked. This allows the second aspect of the present invention to use CO as a feedstock in the fuel synthesis catalyst to synthesize fuel (hydrocarbon of C₅₊) for sufficiently growing carbon chain.

A third aspect of the present invention provides the fuel synthesis device according to the first or second aspect, wherein the supplier further has a function of supplying O₂ gas, and the fuel synthesis device includes: a calculator configured to calculate an amount of O₂ required for oxidation, based on the density of CH₄ detected by the CH₄ density detector; and an O₂ supplier configured to supply the O₂ gas from the supplier to the CH₄ oxidation catalyst, based on the amount of O₂ calculated by the calculator.

O₂ is required to partially oxidize CH₄. The third aspect of the present invention uses the calculator to calculate the amount of O₂ required for oxidation, based on the density of CH₄, and uses the O₂ supplier to supply O₂ to the CH₄ oxidation catalyst from the supplier. This allows the third aspect of the present invention to supply the required amount of O₂ to the CH₄ oxidation catalyst to partially oxidize the required amount of CH₄, without addition of any other specific supplier.

Advantageous Effects of the Invention

The present invention provides a fuel synthesis device to synthesize fuel by effectively utilizing CH₄ produced as a side product.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic configuration of a fuel synthesis device according to a present embodiment;

FIG. 2 is a flowchart of controlling switching between a return path and a bypass path in the fuel synthesis device according to the present embodiment; and

FIG. 3 illustrates a mechanism of synthesizing fuel in Direct-FT.

DETAILED DESCRIPTION

Hereinafter, a description is given in detail of a fuel synthesis device 1 according to an embodiment of the present invention, with reference to the drawings as required. Note that the wordings of upstream and upstream side, and downstream and downstream side in the following description respectively represent an upstream side and a downstream side in a flow direction of fluid flowing into a described device. FIG. 1 of the referenced drawings shows a schematic configuration of the fuel synthesis device 1 according to the present embodiment.

First described are configurations of paths of the fuel synthesis device 1. The fuel synthesis device 1 has a main path 10 through which fuel is synthesized from supplied gas and separated, and a return path 20 through which the gas remaining in a gas phase after the fuel having been separated is returned to an upstream side of a fuel synthesis catalyst 3 in the main path 10. The main path 10 has a piping 11, a piping 12, a piping 13, a piping 14, and a piping 15. Note that the fuel synthesis device 1 has an ECU (Electronic Control Unit) 60 outside these paths. The ECU 60 controls a switching valve V and various other adjustment valves, to be described below, based on a value from a CH₄ density detector 23 arranged in the return path 20. The ECU 60 also controls a compressor 30, a heater 40, an oil-water separator 5, and the like, based on values from various sensors (not shown).

The fuel synthesis device 1 includes a supplier 2, a fuel synthesis catalyst 3, and a gas-liquid separator 4 on a route of the main path 10, as shown in FIG. 1 . The fuel synthesis device 1 includes the fuel synthesis catalyst 3 located downstream of the supplier 2, and the gas-liquid separator 4 located downstream of the fuel synthesis catalyst 3. The fuel synthesis device 1 also includes the oil-water separator 5 on the route of the main path 10 and located downstream of the gas-liquid separator 4.

The fuel synthesis device 1 also includes the compressor 30 to compress gas and the heater 40 to heat the compressed gas, between the supplier 2 and the fuel synthesis catalyst 3. The compressor 30 and heater 40 are connected to each other via the piping 12 in the main path 10. The compressor 30 compresses the gas to a pressure of 3 MPa. The heater 40 heats the gas to a temperature of 330 to 380° C. Note that the pressure and temperature of the gas can be set as required, based on characteristics of the fuel synthesis catalyst 3 and the like.

The supplier 2 is connected with the compressor 30 via the piping 11 in the main path 10. The supplier 2 supplies carbon dioxide (CO₂) gas and hydrogen (H₂) gas to the main path 10. The supplier 2 is also connected with a CH₄ oxidation catalyst 52 via a piping 25. The supplier 2 supplies oxygen (O₂) gas to the piping 25.

The CO₂ gas is supplied from a tank 21 storing CO₂ gas. The ECU 60 controls the supplied amount of the CO₂ gas. Note that the CO₂ gas may be CO₂ in exhaust gas, exhausted from the internal combustion engine of a vehicle such as a car, and/or in atmosphere adsorbed by an adsorbent. In this case, the absorbent may be used as a CO₂ tank with CO₂ desorbed as required.

H₂ gas may be obtained by electrolyzing water in an electrolysis tank 22, with the water being produced by a fuel cell or the like. H₂ gas obtained in this way may be stored in an H₂ tank (not shown) and supplied from the H₂ tank for use. Alternatively, H₂ gas may be supplied from a separate H₂ cylinder if required. The amount of H₂ gas to be supplied is controlled by the ECU 60. Note that electrolyzing water in the electrolysis tank 22 produces O₂. That is, the supplier 2 has a function of supplying O₂ gas. The fuel synthesis device 1 uses the O₂ gas produced in the supplier 2 for partial oxidation of CH₄.

The fuel synthesis catalyst 3 is located downstream of the supplier 2 and chemically reacts CO₂ gas and H₂ gas to synthesize fuel. The fuel to be synthesized is a hydrocarbon having five or more carbons (C₅₊), for example, particularly gasoline. The fuel synthesis catalyst 3 is arranged in a reaction tube 31 in the main path 10. The reaction tube 31 is connected, on an upstream side thereof, to the piping 13 of the main path 10 and connected, on a downstream side thereof, to the piping 14 of the main path 10. The gas supplied from the upstream side of the fuel synthesis catalyst 3 in the main path 10 contains CO₂ and H₂. In the reaction tube 31, the CO₂ and H₂ undergo a chemical reaction (hydrogenation reaction) in a predetermined ratio. For example, an Na—Fe₃O₄/HZSM-5 catalyst may be used as the fuel synthesis catalyst 3, but is not limited thereto. The Na—Fe₃O₄/HZSM-5 catalyst is assumed to catalyze reverse water gas shift (RWGS) reaction on Fe₃O₄, FT synthesis reaction on Fe₅C₂, and oligomerization, isomerization, and aromatization at acid points on zeolite. When the Na—Fe₃O₄/HZSM-5 catalyst is used, hydrocarbons of C₅ to C₁₁ are obtained in maximum yield of 78%, with low production of methane (CH₄) and CO.

Fuel synthesis by the fuel synthesis catalyst 3 is executed by using a known technique. For example, H₂ metered to have a predetermined ratio of CO₂ to H₂ in the reaction tube 31 is supplied from the electrolysis tank 22 or the H₂ tank (not shown) to the piping 11, and the gas in the reaction tube 31 is then compressed and heated by the compressor 30 and heater 40. This causes the aforementioned RWGS reaction, FT synthesis reaction, and oligomerization and other reactions to proceed in the reaction tube 31, under the action of the fuel synthesis catalyst 3, to produce hydrocarbons (gasoline) of C₅ to C₁₁ as fuel (Equation (4) below).

nCO₂+mH₂→C_(n)H_(2(m−2n))+2nH₂O   (4)

The gas-liquid separator 4 is located downstream of the fuel synthesis catalyst 3, particularly between the piping 14 and piping 15 in the main path 10, and cools the fuel to a liquid and separates the liquid from the gas containing the CO₂ gas and H₂ gas, which have not been reacted in the fuel synthesis catalyst 3, and CH₄ gas as a side product. The gas-liquid separator 4 cools the synthesis gas containing gasoline by heat exchange and condenses the gas, to separate the above-described unreacted gas (gas phase) from the gasoline-based liquid (liquid phase (hydrocarbons of C₅₊)). Alternatively, the gas-liquid separator 4 may separate the synthesis gas through membrane separation, to separate the gasoline-based liquid (liquid phase). The gasoline-based liquid phase, which has been separated by the gas-liquid separator 4, is fed through the piping 15 to the oil-water separator 5. The gas phase separated by the gas-liquid separator 4 contains hydrocarbons of CH₄ and C₂₋₄ as side products, unreacted CO₂ and H₂, and unrecovered gasoline and water (H₂O). The gas separated as gas phase in the gas-liquid separator 4 is fed through the return path 20 to a point upstream of the fuel synthesis catalyst 3, such as to the compressor 30, and used again for fuel synthesis.

The oil-water separator 5 separates gasoline and water in the liquid phase from each other, using the difference in boiling points. The oil-water separator 5 heats the liquid phase to 35° C. or more but less than 100° C., for example. This allows for obtaining gasoline evaporated from the liquid phase, which is either left as gas or liquefied and fed to a fuel tank, not shown, through the piping 16. The remaining liquid phase is almost entirely water (H₂O) and is discharged outside through the piping 17. Note that the heating temperature is desirably set such as to 40° C. or more, 50° C. or more, and 60° C. or more, from the viewpoint of evaporating gasoline. Additionally, the heating temperature may be set such as to 90° C. or less, 80° C. or less, and 70° C. or less, from the viewpoint of reducing water contamination. Alternatively, the heating temperature may be set to multiple temperature ranges in consideration of fractional distillation of gasoline.

In the present embodiment, a bypass path 50 is provided in the return path 20. The bypass path 50 bypasses, and merges downstream of, the return path 20. The bypass path 50 includes, on a route thereof, a CH₄ separator 51 to separate CH₄ and the CH₄ oxidation catalyst 52 to oxidize CH₄ separated by the CH₄ separator 51.

For example, a molecular sieve may be used as the CH₄ separator 51. The CH₄ transmitting through, and separated by, the CH₄ separator 51 is fed via a piping 53 to the CH₄ oxidation catalyst 52. Note that the gas phase transmitting through, and separated by, the CH₄ separator 51(transmitted gas phase) may contain H₂, and this H₂ is also fed to the CH₄ oxidation catalyst 52, along with CH₄. In contrast, the gas phase which has not transmitted through the CH₄ separator 51 (retained gas phase) may contain hydrocarbons of CO₂ and C₂₋₄. The hydrocarbons of CO₂ and C₂₋₄ are fed to the return path 20 via a piping 54. These hydrocarbons of CO₂ and C₂₋₄ are then fed to a point upstream of the fuel synthesis catalyst 3, such as the compressor 30, and are used again for fuel synthesis.

The CH₄ oxidation catalyst 52 causes CH₄ to be incompletely com busted at an oxygen density lower than the theoretical oxygen content required for complete combustion. For example, a Pd/Al₂O₃ catalyst may be used as the CH₄ oxidation catalyst 52. The Pd/Al₂O₃ catalyst reacts CH₄ and O₂ in a predetermined ratio under conditions of 400° C., for example, to partially oxidize CH₄ to produce CO and H₂ (Equation (5)).

CH₄+½O₂→CO+2H₂   (5)

The CO and H₂, produced as above, and the H₂, fed to the CH₄ oxidation catalyst 52 as being unreacted, are fed to the return path 20 via a piping 55. The CO and H₂ are then fed to the point upstream of the fuel synthesis catalyst 3, such as the compressor 30, along with the hydrocarbons of CO₂ and C₂₋₄, and are used again for fuel synthesis.

The switching valve V is provided at the junction of the return path 20 and the bypass path 50. The switching valve V selectively switches between communication with the return path 20 and communication with the bypass path 50.

In the return path 20, a CH₄ density detector 23 is placed between the gas-liquid separator 4 and the switching valve V. The CH₄ density detector 23 detects density of CH₄ contained in the gas separated by the gas-liquid separator 4. The CH₄ density detector 23 can be an HC analyzer or an exhaust gas detector to detect the density of hydrocarbons. The CH₄ density detector 23 outputs density (detected value) of the CH₄ to the ECU 60. The ECU 60 includes a calculator (not shown) to calculate, based on the inputted density of CH₄, an amount of O₂ required for partially oxidizing the CH₄ in the CH₄ oxidation catalyst 52. The calculator is implemented by a CPU (Central Processing Unit, not shown) of the ECU 60 executing a program necessary for the calculation. The ECU 60 then outputs the required amount of O₂ calculated above to a mass flow controller MFC.

The CH₄ oxidation catalyst 52 is connected with the supplier 2 (particularly the electrolysis tank 22) via the piping 25 as described above. The O₂ generated in the electrolysis tank 22 is fed via the piping 25 to the CH₄ oxidation catalyst 52 (O₂ supplier). The piping 25 is provided with the mass flow controller MFC. The mass flow controller MFC meters mass flow of O₂ for flow rate control. The mass flow controller MFC feeds O₂ to the CH₄ oxidation catalyst 52, while controlling a flow rate of O₂ supplied from the electrolysis tank 22, based on the amount of O₂ calculated by the ECU 60. This allows for suitably executing partial oxidation of CH₄ in the CH₄ oxidation catalyst 52.

In the present embodiment, whether the switching valve V communicates with the return path 20 or the bypass path 50 is controlled based on the density of CH₄ detected by the CH₄ density detector 23. The ECU 60 controls the switching valve V. In this way, the fuel synthesis device 1 controls the switching valve V based on the density of CH₄ remaining in the device to partially oxidize the CH₄ and returns the gas (CO) produced by oxidation to the point upstream of the fuel synthesis catalyst 3 (between the supplier 2 and the fuel synthesis catalyst 3).

This allows the fuel synthesis device 1 to use CO as a feedstock in the fuel synthesis catalyst 3, to synthesize fuel (hydrocarbon of C₅₊) for sufficiently growing carbon chain. That is, the fuel synthesis device 1 effectively utilizes CH₄ produced as a side product in the fuel synthesis catalyst 3, to synthesize fuel. The fuel synthesis device 1 uses carbon dioxide to synthesize fuel (gasoline), resulted in reducing carbon dioxide and its negative impact on the global environment.

The fuel synthesis device 1 may cause the switching valve V to communicate with the bypass path 50, on the condition that the density of CH₄ detected by the CH₄ density detector 23 becomes equal to or greater than a predetermined value at which synthesis of the fuel is blocked. The predetermined value may suitably be set in consideration of performance of the fuel synthesis catalyst 3.

In such an embodiment, the ECU 60 of the fuel synthesis device 1, on the condition that density of the CH₄ remaining in the device increases to become equal to or greater than the predetermined value at which synthesis of the fuel is blocked, causes the switching valve V to be switched to communicate with the bypass path 50 to allow the CH₄ to be partially oxidized, so that the gas (CO) produced by oxidation is returned to the point upstream of the fuel synthesis catalyst 3. Accordingly, the fuel synthesis device 1 uses CO as a feedstock in the fuel synthesis catalyst to synthesize fuel (hydrocarbon of C₅₊) for sufficiently growing carbon chain. Note that the ECU 60 in this embodiment, on the condition that the density of the CH₄ is less than the predetermined density, causes the switching valve V to remain unmoved to keep communicating with the return path 20. The switching valve V communicates with the return path 20 at a time of starting operation and during normal operation. The unreacted CO₂ and H₂, CH₄ as a side product, and hydrocarbon of C₂₋₄ are then fed (returned) to the point upstream of the fuel synthesis catalyst 3, particularly to the compressor 30. In this way, the CH₄ oxidation catalyst 52 is not used to prevent the CH₄ oxidation catalyst 52 from being degraded, so that the life of the CH₄ oxidation catalyst 52 is extended.

Next, a description is given of a preferable embodiment of controlling switching between the return path 20 and the bypass path 50 in the fuel synthesis device 1 of the present embodiment, with reference to FIG. 2 . FIG. 2 is a flowchart of controlling switching between the return path 20 and the bypass path 50 in the fuel synthesis device 1 of the present embodiment.

The fuel synthesis device 1 starts operation, as shown in FIG. 2 . The supplier 2, the compressor 30, the heater 40, the fuel synthesis catalyst 3, the gas-liquid separator 4, and the oil-water separator 5 operates as described above, to produce hydrocarbon of C₅₊ as fuel. The fuel synthesis catalyst 3 produces CH₄ as a side product, in association with fuel being produced.

Then, the CH₄ density detector 23 detects density of CH₄ as a side product, which has been separated by the gas-liquid separator 4, and outputs the density of CH₄ to the ECU 60 (step S1). The ECU 60 determines whether the density of CH₄ detected by the CH₄ density detector 23 is less than the predetermined value, or equal to or greater than the predetermined value (step S2).

The ECU 60, on the condition that the density of CH₄ has been less than the predetermined value in step S2, causes the switching valve V to remain unmoved to keep communicating with the return path 20 (step S3). In this case, there is no need of supplying O₂ to the CH₄ oxidation catalyst 52, so that the ECU 60 controls the mass flow controller MFC to shut off the flow (step S4). The processing then returns to step S1 so that the fuel synthesis device 1 operates and the CH₄ density detector 23 detects the density of CH₄ as a side product.

In contrast, the ECU 60, on the condition that the density of CH₄ is equal to or greater than the predetermined value, causes the switching valve V to communicate with the bypass path 50 (step S5). Concurrently in step S2, the ECU 60 causes the calculator to calculate an amount of O₂ required for partially oxidizing CH₄ in the CH₄ oxidation catalyst 52, based on the density of CH₄, and outputs the required amount of O₂ calculated above to the mass flow controller MFC. The ECU 60 then causes the mass flow controller MFC to operate to supply O₂ to the CH₄ oxidation catalyst 52, while controlling the flow rate of O₂ supplied from the electrolysis tank 22, based on the amount of O₂ calculated by the ECU 60 (step S6). Upon the required amount of O₂ having been supplied, the processing returns to step S1 so that the fuel synthesis device 1 operates and the CH₄ density detector 23 detects the density of CH₄ as a side product.

As described hereinabove, the fuel synthesis device 1 of the present embodiment controls whether the switching valve V communicates with the return path 20 or the bypass path 50, based on the density of CH₄ detected by the CH₄ density detector 23. The fuel synthesis device 1 thus controls the switching valve V based on the density of CH₄, staying in the device, to cause CH₄ to be partially oxidized and return gas (CO) produced by oxidation to the point upstream of the fuel synthesis catalyst 3 (between the supplier 2 and the fuel synthesis catalyst 3). This allows the fuel synthesis device 1 to use CO as a feedstock in the fuel synthesis catalyst 3 to synthesize fuel (hydrocarbon of C₅₊) for sufficiently growing carbon chain.

The present invention is not limited to the above-described embodiment and can be implemented in various embodiments. In addition, said embodiments can be combined to the extent structurally feasible.

LIST OF REFERENCE SIGNS

1: fuel synthesis device, 2: supplier, 3: fuel synthesis catalyst, 4: gas-liquid separator, 5: oil-water separator, 10: main path, 11 to 17: piping, 20: return path, 21: tank, 22: electrolysis tank, 23: CH₄ density detector, 25: piping, 30: compressor, 31: reaction tube, 40: heater, 50: bypass path, 51: CH₄ separator, 52: CH₄ oxidation catalyst, 53 to 55: piping, 60: ECU, MFC: mass flow controller, and V: switching valve. 

What is claimed is:
 1. A fuel synthesis device comprising: a supplier arranged upstream of a main path and configured to supply CO₂ gas and H₂ gas; a fuel synthesis catalyst located downstream of the supplier and configured to chemically react the CO₂ gas and the H₂ gas to synthesize fuel; a gas-liquid separator arranged downstream of the fuel synthesis catalyst and configured to liquefy the fuel into liquid and separate the liquid from a gas containing the CO₂ gas and the H₂ gas, which have not yet reacted with the fuel synthesis catalyst, and gas of CH₄ as a side product; a return path configured to return the gas separated by the gas-liquid separator to a point in a flow path between the supplier and the fuel synthesis catalyst; a bypass path configured to bypass, and merge downstream of, the return path, and including a CH₄ separator to separate the CH₄ and a CH₄ oxidation catalyst to oxidize the CH₄ separated by the CH₄ separator; a switching valve provided at a junction of the return path and the bypass path and configured to selectively switch between communication with the return path and communication with the bypass path; and a CH₄ density detector arranged in the return path and configured to detect density of CH₄ contained in the gas separated by the gas-liquid separator, wherein whether the switching valve communicates with the return path or the bypass path is controlled based on the density of CH₄ detected by the CH₄ density detector.
 2. The fuel synthesis device according to claim 1, wherein the switching valve is controlled to communicate with the bypass path, on the condition that the density of CH₄ detected by the CH₄ density detector becomes equal to or greater than a predetermined value at which synthesis of the fuel is blocked.
 3. The fuel synthesis device according to claim 1, wherein the supplier further has a function of supplying O₂ gas, and the fuel synthesis device includes: a calculator configured to calculate an amount of O₂ required for oxidation, based on the density of CH₄ detected by the CH₄ density detector; and an O₂ supplier configured to supply the O₂ gas from the supplier to the CH₄ oxidation catalyst, based on the amount of O₂ calculated by the calculator. 